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(#) Lambda Parameters Last

!!! WARNING: Lambda Parameters Last
   This is a warning.

Id
:   `LambdaLast`
Summary
:   Lambda Parameters Last
Note
:   **This issue is disabled by default**; use `--enable LambdaLast`
Severity
:   Warning
Category
:   Interoperability: Kotlin Interoperability
Platform
:   Any
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   3.2.0 (September 2018)
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
See
:   https://android.github.io/kotlin-guides/interop.html#lambda-parameters-last
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/InteroperabilityDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/InteroperabilityDetectorTest.kt)

To improve calling this code from Kotlin, parameter types eligible for
SAM conversion should be last.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/pkg/Test.java:11:Warning: Functional interface parameters (such
as parameter 1, "run", in test.pkg.Test.error1) should be last to
improve Kotlin interoperability; see
https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions
[LambdaLast]
    public void error1(Runnable run, int x) { }
                                     -----
src/test/pkg/Test.java:12:Warning: Functional interface parameters (such
as parameter 1, "sam", in test.pkg.Test.error2) should be last to
improve Kotlin interoperability; see
https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions
[LambdaLast]
    public void error2(SamInterface sam, int x) { }
                                         -----
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here are the relevant source files:

`src/test/pkg/Test.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package test.pkg;
@SuppressWarnings("ClassNameDiffersFromFileName")
public class Test {
    public void ok1() { }
    public void ok1(int x) { }
    public void ok2(int x, int y) { }
    public void ok3(Runnable run) { }
    public void ok4(int x, Runnable run) { }
    public void ok5(Runnable run1, Runnable run2) { }
    public void ok6(java.util.List list, boolean b) { }
    public void error1(Runnable run, int x) { }
    public void error2(SamInterface sam, int x) { }

    public interface SamInterface {
        void samMethod();
        @Override String toString();
        default void other() {  }
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

`src/test/pkg/test.kt`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~kotlin linenumbers
package test.pkg

fun ok1(bar: (Int) -> Int) { }
fun ok2(foo: Int) { }
fun ok3(foo: Int, bar: (Int) -> Int) { }
fun ok4(foo: Int, bar: (Int) -> Int, baz: (Int) -> Int) { }
// Lamda not last, but we're not flagging issues in Kotlin files for the
// interoperability issue
fun error(bar: (Int) -> Int, foo: Int) { }
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/InteroperabilityDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `InteroperabilityDetector.testLambdaLast`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("LambdaLast")
  fun method() {
     problematicStatement()
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("LambdaLast")
  void method() {
     problematicStatement();
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection LambdaLast
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="LambdaLast" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'LambdaLast'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore LambdaLast ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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